The present invention relates to a process for the purification of aromatic amines which are obtained by reduction of aromatic nitro compounds. Aromatic amines as used herein means those compounds which carry at least one amino group on an aromatic ring. The latter may be substituted or fused with other aromatic rings.
Aromatic amines are important intermediates which must be available inexpensively and in large amounts. For the preparation of aniline, for example, installations with very high capacities must therefore be built. Aniline is, for example, an important intermediate in the preparation of methylenediphenyl-diisocyanate (MDI) and is prepared on a large industrial scale as a rule by catalytic reduction of nitrobenzene with hydrogen, as described, e.g., in DE-A 2201528; DE-A 3414714; U.S. Pat. No. 3,136,818; EP-B1 0 696 573; and EP-B1 0 696 574.
In catalytic reductions for the preparation of aromatic amines, water and organic secondary components are formed in addition to the desired product(s). These organic secondary components (“by-products”) must be separated off before further use of the aromatic amine(s). These by-products include “low-boiling substances”, i.e. compounds or azeotropically boiling mixtures of individual components having boiling points below that of the amine to be prepared, and “high-boiling substances”, i.e. compounds or azeotropically boiling mixtures of individual components having boiling points above that of the amine to be prepared. In the case of the reduction of nitrobenzene to aniline (b.p.=184° C.), examples are benzene (b.p.=80° C.) for the group of low-boiling substances and diphenylamine (b.p.=302° C.) for the group of high-boiling substances. These two impurities mentioned as examples can be easily separated off by distillation because their boiling points are very different from that of the amine to be prepared (ΔTB=104 K and 118 K respectively). Separation of those secondary components which have boiling points very similar to that of the amine to be prepared is considerably more problematic, because the outlay on distillation will be considerably higher. In the case of the reduction of nitrobenzene, the separation of phenol (b.p.=182° C.) and the target product aniline (b.p.=184° C.) in particular is a great challenge for distillation technology. The difficulty of this separation is evident from the fact that long distillation columns with a high number of separating stages and high reflux ratios with correspondingly high outlay on investment and energy are used. Compounds with phenolic hydroxyl groups, i.e. compounds which carry at least one hydroxyl group on an aromatic ring, can generally be problematic in the working up of aromatic amines. In the case of aniline, in addition to the phenol already mentioned, the various aminophenols are also problematic.
Therefore, the purification of aromatic amines is not trivial, and is of great industrial importance. Recent approaches are directed, in particular, to resolving the problems mentioned in connection with compounds having phenolic hydroxyl groups. One approach is to convert the compounds with phenolic hydroxyl groups into the corresponding salts by reaction with suitable bases. The salts are non-volatile compounds which are considerably easier to separate off. For this purpose, alkali metal hydroxides are employed for an extraction, or an alkali metal hydroxide is added during the distillation.
JP-A-49-035341 describes a process in which the amine to be purified, aniline, is brought into contact with solid alkali metal hydroxides in a fixed bed and only then is passed into the distillation, or the distillation is carried out in the presence of the solid alkali metal hydroxide in amounts of 0.1-3 percent by weight, based on the amount of aniline to be distilled. The separating off of critical components, such as the aminophenols, is simplified by this means. However, disadvantages of this process are the use of high molar excesses of the solid alkali metal hydroxides in relation to the acidic secondary components to be removed and the impossibility of precise metering of the alkaline compounds. This can lead to corrosion problems, precipitates and high-viscosity bottom product phases in the distillation column in the event of over-metering, and to an incomplete removal of the critical components in the event of under-metering.
JP-A-08-295654 describes, as an alternative to removal of compounds with phenolic hydroxyl groups from aniline by distillation, an extraction with dilute aqueous sodium hydroxide solution or potassium hydroxide solution (concentration 0.1-0.7 percent by weight, based on the weight of the alkali metal hydroxide solution) by which most of the phenol is transferred as alkali metal phenolate into the aqueous phase, and the phenolate is separated off by the subsequent phase separation. For effective reduction of the phenol content, a molar ratio of NaOH:phenol in the range of 3:1-100:1 is required. Disadvantages of this process are the high NaOH consumption (molar excesses), the production of very large amounts of waste water containing alkali metal phenolate—as a result of the low concentration of the alkali metal hydroxide solutions—which leads to additional disposal costs, and an additional outlay on investment for the extraction.
US-A-2005 080294 describes a process for separating off compounds with phenolic hydroxyl groups (“phenolic compounds”) from aromatic amines, in which before the distillation a base is added to the amine to be purified in a molar ratio of from 1:1 to 4:1, based on the “phenolic compounds”, optionally in the presence of polyols. US-A-2005 080294 does not teach in detail what happens to the salts which are formed in the reaction of the “phenolic compounds” with the bases. In Example 6, it is merely mentioned that excess solid KOH is dissolved by addition of polyethylene glycol (PEG). What consequences are associated with this is not to be found in US-A-2005 080294. US-A-2005 080294 does not go into detail at all with respect to the salts of the “phenolic compounds” themselves.
However, salts, excess base, and the salts of the compounds with phenolic hydroxyl groups, in general are only sparingly soluble in aromatic amines, so that there is great danger that they will become concentrated beyond the solubility limit in the distillation column, in the bottom product of the distillation column and/or in the evaporator of the distillation, and then precipitate out. Such solid precipitates can severely interfere with the distillation process, so that an interruption in the distillation operation becomes necessary. Such interruption can lead to considerable difficulties and even to losses in production in large-scale industrial processes continuously. US-A-2005 080294, however, is not concerned with the problem of reliability and service life of the process. The person skilled in the art also does not learn from US-A-2005 080294 that the presence of the salts formed during the reaction of the compounds with phenolic hydroxyl groups with the bases can lead to deposition of solids, fouling and/or a high increase in viscosity during the distillation. US-A-2005 080294 does not go into details of the distillation technique at all. The person skilled in the art therefore does not learn from US-A-2005 080294 how he is to solve these problems which occur with high probability. US-A-2005 080294 teaches only the optional addition of PEG in order to dissolve excess solid KOH. Such an addition of PEG into the distillation, however, is economically unacceptable because of the high capacities in the preparation of aromatic amines (in particular aniline). The use of the process described in US-A 2005 080294 in a continuous production process is not described.
JP-A-2005 350388 is concerned quite generally with improving the working up of aniline. A process is described in which some of the bottom product of the aniline distillation column is removed and transferred into the gas phase separately, i.e. in a second evaporator which differs from the actual evaporator of the column. The gas phase obtained in this way is recycled into the pure aniline column; high-boiling contents which cannot be evaporated are separated off. In this manner, the temperature of the aniline distillation column can be kept relatively low and amounts of impurities can be reduced; the aniline loss in the separating off of high-boiling substances is likewise reduced. A disadvantage of this process is that before the actual aniline distillation column, low-boiling substances and water are separated off separately by an additional distillation in a dewatering column in a process which is expensive in terms of apparatus. JP-A-2005 350388 does not mention the particular problems with compounds with phenolic hydroxyl groups for the distillation. It is therefore also not to be ascertained from this Japanese patent application whether separating these off from the target product aniline can be improved with the process described there.
EP-A-07075103 describes a process for the purification of aniline by addition of an aqueous alkali metal hydroxide solution before or during the distillation. EP-A-07075104 describes a process for the purification of aniline by extraction with aqueous alkali metal hydroxide solution. In contrast to the present Application, neither of these published disclosures teaches partial or complete sluicing out of the bottom product of the aniline distillation column and partial evaporation thereof via two evaporators (E1) and (E2) connected in series or parallel. By the procedure of the present invention described in more detail herein after, a maximum depletion of the valuable amine in the bottom product of the distillation column is achieved with a minimum outlay on apparatus and energy.